Some embodiments relate to optical waveguides. More precisely, some embodiments relate to a method for manufacturing photonic waveguides and to photonic waveguides manufactured by this method.
Photonic waveguides have attracted much attention over the last thirty years, because of their potential ability to confine electromagnetic waves to reinforce non-linear electro-optical interactions.
Photonic waveguides are optical waveguides formed on thin dielectric substrates of a thickness between a few hundred nanometers and a few microns, and in which light is confined laterally either by etching material or by an index gradient.
One known method for manufacturing a photonic waveguide includes thinning a dielectric substrate by applying the ion implantation technique, and then forming an optical waveguide in the thinned dielectric substrate. FIGS. 1a-1g illustrate one example of this method.
In particular, FIG. 1a illustrates a dielectric substrate 10 having a first surface S1, a second surface S2 and a thickness R before thinning with the ion implantation technique. FIG. 1b illustrates the dielectric substrate 10 after ions have been implanted at a distance R′ from the surface S2. Particularly, in FIG. 1b, the portion of the dielectric substrate 10 having a thickness R′, which portion is defined by the dashed line, corresponds to the portion of the dielectric substrate 10 that has seen ion implantation. Next, as illustrated in FIG. 1c, the surface S2 of the dielectric substrate 10 is bonded or joined to another dielectric substrate 20, of a thickness X, which is known as the superstrate. Next, a step of cutting the dielectric substrate 10 is carried out in such a way that the superstrate 20 and the portion of the dielectric substrate 10, of a thickness R′, that has been implanted with ions are separated from the portion of the dielectric substrate 10 of thickness R-R′ (see FIG. 1d). FIG. 1e illustrates the portion of the dielectric substrate 10, of a thickness R′, that remains after the dielectric substrate 10 has been thinned with the ion implantation technique. This remaining portion of the dielectric substrate 10, which has a thickness R′ and which is bonded to the superstrate 20, corresponds to a thin dielectric substrate of a thickness between a few hundred nanometers and a few microns. This thin dielectric substrate resulting from the thinning of the dielectric substrate 10 forms a planar optical waveguide. Next, a microstrip optical waveguide is formed on the free surface of the dielectric substrate 10 having a thickness R′, either via a step of ion exchange (H+ for example, or Na+ in glass) through a mask deposited beforehand, or by etching (dry ionic plasma etching or wet etching with hydrofluoric acid or by cutting with a precision circular saw) through a mask deposited beforehand.
In particular, FIG. 1f illustrates a perspective view of a microstrip photonic waveguide in which the portion of the dielectric substrate 10 (of a thickness R′) remaining after the dielectric substrate 10 has been thinned is bonded to the superstrate 20. Furthermore, FIG. 1f illustrates the microstrip 4 of the microstrip optical waveguide formed in the remaining portion of the dielectric substrate 10 after the aforementioned ion-exchange step.
It will be noted that the etching (by ionic reactive plasma etching or by wet etching or by cutting-polishing with a circular saw) of the dielectric substrate 10 may be such that an optical waveguide having a “ridge” structure is formed on the dielectric substrate 10 instead of a microstrip optical waveguide. In this case, the photonic waveguide obtained is known as a ridge photonic waveguide.
In particular, FIG. 1g illustrates a perspective view of a ridge photonic waveguide in which the portion of the dielectric substrate 10 (of a thickness R′) remaining after the dielectric substrate 10 has been thinned is bonded to the superstrate 20. Furthermore, FIG. 1g illustrates the ridge optical waveguide 6 formed in the portion of the dielectric substrate 10 remaining after the aforementioned etching. It will be noted that the ridge optical waveguide 6 may have on its surface a strip having a high refractive index. This high-refractive-index strip is represented by the hatched region of the surface of the ridge optical waveguide 6 in FIG. 1g. This region may for example be obtained by depositing a high-index dielectric prior to the etching, or by carrying out an ion exchange prior to the etching, in such a way that the ridge optical waveguide 6 ends up with a high-index strip on its surface.
Thus, FIGS. 1f and 1g show a photonic waveguide that includes a microstrip optical waveguide and a photonic waveguide that includes a ridge optical waveguide, respectively, these two waveguides being manufactured by the aforementioned known method, in which method the thinning of a dielectric substrate is achieved by implying the ion implantation technique.
It will be noted that one example of the aforementioned technique for thinning the dielectric substrate by ion implantation is slice cutting thin microcrystalline films (i.e. the “SmartCut” or “ion slicing” technique). This example is described in the publication by G. Poberaj et al. “Ion-sliced lithium niobate thin films for active photonic devices, Optical Materials, 31, 1054-1058 (2009)”. Another example of a technique for thinning a dielectric substrate by ion implantation is ion-beam enhanced etching, which is described in the publication by R. Geiss et al. “Light propagation in a free-standing lithium niobate photonic crystal waveguide, Applied Physics Letters, 97, 131109 (2010)”. In particular, these techniques consist in implanting ions (e.g. helium ions, protons or argon ions) at high-energy on a surface of a dielectric substrate and then in carrying out a chemical treatment (in hydrofluoric acid) or a heat treatment of this surface.
As mentioned above, the thickness R′ of the portion of the dielectric substrate remaining after the dielectric substrate has been thinned with the ion implantation technique is between a few hundred nanometers and a few microns.
It has been observed that when the thickness R′ is below 5 μm, a strong vertical confinement of the electromagnetic wave guided in the optical waveguide is obtained but at the same time substantial insertion losses (higher than 3 dB) appear between the photonic waveguide and a standard single-mode optical fiber (of SMF28 type) coupled to the photonic waveguide.
It is known that insertion losses correspond to the contribution of coupling losses between the photonic waveguide and the standard single-mode optical fiber and of propagation losses of the electromagnetic wave over the length of the photonic waveguide. In the case of a thickness R′ smaller than 5 μm, the coupling losses per facet with a standard single-mode fiber are typically higher than 3 dB. This is related to the low overlap between the confined optical mode of the photonic waveguide and the weakly confined optical mode of the standard single-mode fiber. These coupling losses per facet even exceed 10 dB if the thickness R′ is smaller than 1 μm. Moreover, the propagation losses are higher than 0.7 dB/cm in the case where the thickness R′ is smaller than 5 μm and these losses even exceed 2 dB/cm if the thickness R′ is smaller than 1 μm.
It will be noted that vertical and/or lateral transition zones have been proposed to mitigate the problem of high insertion losses in photonic waveguides formed by the aforementioned thinning method. These transition zones consist in varying continuously and gradually the lateral or vertical dimension of the photonic waveguide at the end of the latter, so as to match the optical mode of the photonic waveguide to the weakly confined optical mode of the standard single-mode fiber. The gradual variation in the lateral or vertical dimension of the photonic waveguide occurs on the surface in which the microstrip optical waveguide or the ridge optical waveguide is formed. It will be noted that such a variation in guide height disrupts the guided electromagnetic wave, because optical modes other than the fundamental optical mode are also excited. Moreover, the production of these transition zones implies even more manufacturing steps (at the minimum at least one etching or deposition step in addition to the aforementioned steps in the known method for manufacturing a photonic microguide).
Furthermore, it will be noted that the ion implantation technique mentioned above with regard to the formation of photonic waveguides is very onerous to implement, and in particular energies of the order of a MeV must be provided for the implantation of the ions. Moreover, energies of the order of a MeV imply constraining safety conditions (buildings subject to strict standards, a medical visit before the start of work and periodic follow-up inspections).
Furthermore, it will be noted that thin dielectric substrates (of a thickness of 5 μm or less) bonded to a superstrate do not resist temperatures above 800° C., this preventing heat treatments such as diffusions or anneals from being carried out on the photonic waveguide. However, as is known to those skilled in the art, such a heat treatment may be important, in particular to decrease the roughness of the photonic waveguide and therefore propagation losses, or to measure high temperatures using the photonic waveguide.
There is therefore a real need to provide a method for manufacturing a photonic waveguide, which allows very thin dielectric substrate thicknesses, possibly smaller than 5 μm, to be obtained without substantial insertion losses between the photonic waveguide and a standard single-mode optical fiber, which at the same time is simple to implement without involving constraining safety conditions and without exciting optical modes other than the fundamental optical mode, and which allows photonic waveguides that are robust to high temperatures to be produced.